This invention pertains generally to the field of semiconductor devices and to quantum computing carried out in semiconductor devices.
Quantum computing utilizes quantum particles to carry out computational processes. The fundamental unit of quantum information is called a quantum bit or qubit. A qubit can be both a zero and a one at the same time. An example is the spin of an electron, wherein the up or down spin can correspond to a zero, a one, or a superposition of states in which it is both up and down at the same time. Performing a calculation using the electron essentially performs the operation simultaneously for both a zero and a one. Experimental advances in quantum computation have come most rapidly in nuclear magnetic resonance (NMR) and ion-trap systems. The success of few-qubit quantum computation in such systems demonstrates an urgent need for a quantum computing scheme that is scaleable to a large number of qubits. Solid-state qubits are one of the primary candidates. Numerous proposals have been made for solid-state quantum computers. These proposals include the use of nuclear spins as qubits, B. E. Kane, xe2x80x9cA Silicon-Based Nuclear Spin Quantum Computer,xe2x80x9d Nature, Vol. 393 (6681), 1998, pp. 133-137; and the use of electronic spins as quantum dots, DiVincenzo, et al., xe2x80x9cQuantum Computers and Quantum Coherence,xe2x80x9d J. of Magnetism and Magnetic Materials, Vol. 200, (1-3), 1999, pp. 202-218. Potential issues with such proposed systems include individual impurity spins, as well as gate operation and readout methods for the quantum dots.
Spins can be manipulated using a strong DC magnetic field combined with a spatially uniform radio frequency (i.e., at GHz frequencies) field. In the presence of a small g-factor gradient, the spins can be addressed individually. Entanglement of one spin with another proceeds by gating the barrier between spins. This gives rise to a time-dependent exchange interaction, H(t)=J(t)S1xc2x7S2. A combination of these operations acting in the proper sequence on two qubits will produce a controlled-NOT gate (C-NOT). See, e.g., R. Vrijen, et al., xe2x80x9cElectron-Spin Resonance Transistors for Quantum Computing and Silicon-Germanium Heterostructures,xe2x80x9d Physical Review A (Atomic, Molecular, and Optical Physics), Vol. 62 (1), 2000, pp. 012306/1-10.
Quantum computation also can be performed without g-factor tuning and the individual spin rotations via high frequency radiation that g-factor tuning allows. Instead, the time-dependent exchange interaction, H(t)=J(t)S1xc2x7S2, can be used in combination with coded qubits, D. P. DiVincenzo, D. Bacon, J. Kempe, G. Burkard, K. B. Whaley, Nature (London) 408, 339 (2000), in which a single qubit is represented by the total wavefunction of several individual spins. In this way, the exchange interaction alone enables universal quantum computation.
In accordance with the present invention, electron quantum dot semiconductor devices may be utilized for such purposes as quantum computing, quantum memory, and quantum information processing. The invention may be implemented in a semiconductor heterostructure to trap individual electrons in a solid, bring these electrons close to each other, maintain phase coherence of the electrons, and allow individual rotation of the spin of the electrons.
A semiconductor quantum dot device in accordance with the invention includes a multiple layer semiconductor structure having a quantum well, a back gate electrode, and a plurality of spaced surface electrode gates. The electrode gates are spaced from each other by a region beneath which quantum dots may be defined. A tunnel barrier layer is provided between the back gate and the quantum well, and a barrier layer is formed over the quantum well layer. Appropriate voltages applied to the electrodes allow the development and appropriate positioning of the quantum dots. This arrangement in accordance with the invention allows a large number of quantum dots, each containing as few as one electron, to be formed in a series with appropriate coupling between the quantum dots, enabling quantum computation with a large number of qubits. In addition to the embodiment of the present invention in a quantum computer, the invention may also be implemented in other applications such as a single electron transistor and in integrated circuit technologies where control of one or a few electrons is desired.
The heterostructure and split top gates that enable the creation of coupled quantum dots may also be used to channel electrons across an integrated circuit. In this mode of operation, the invention provides a local tuning of the potential in a quantum well to provide channels in which individual electrons can be moved through a series of devices enabling, for example, readout of quantum memory.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.